1. Field of the Invention
This invention applies to tree handling devices for the logging industry, and particularly to heads mounted to forestry carriers for harvesting and processing trees.
2. Description of the Prior Art
Tree harvesting and processing heads are well known in the logging industry and generally perform several operations including felling, delimbing, debarking, bucking, sorting and piling trees, e.g. U.S. Pat. No. 5,785,101 (Wildey). To effect the delimbing, debarking and bucking operations, various means have been used to feed the felled tree longitudinally through the head. The present invention is concerned with those heads employing some form of rotary drive mechanism for tree feeding e.g. hydraulically driven wheels or tracks.
It is well known in the logging industry that improved productivity can be achieved through increased feeding force and increased feeding speed. Increasing the feeding force improves the ability to accelerate and decelerate the tree and improves the ability to delimb heavily branched trees or bunches of trees. Increasing the feeding speed reduces the time required to process each tree or bunch of trees.
The most durable tree-feeding designs have used fixed-displacement hydraulic motors for their rotary drive mechanisms. However, with fixed-displacement motors, either high feeding speed or high feeding force can be achieved, but not both without excessive hydraulic flow and pressure (horsepower). These limitations are especially evident on the smaller, lower horsepower carriers. Using higher horsepower carriers requires more fuel and adds greater overall costs to production. To achieve the desired increases in feeding speed and feeding force while maintaining good efficiency, various methods have been employed in the prior art including variable displacement hydraulic motors, dual-displacement or multiple-displacement hydraulic motors and the like. These designs have proven to lack the durability and simplicity of simple fixed displacement motors and add complexity and cost to the head.
It is also known in the prior art that employing three or more drives in a triangulated or similar arrangement reduces the friction as the tree feeds through the head. Multiple drives also increase the force that can be transmitted to the tree without causing excessive fiber damage. Generally, these multiple drive designs have proven to be superior, especially in heavily branched trees. Heads with only two drives usually employ one or more idler or free-wheeling wheels for the tree to slide against as it is being fed, to approximate the reduced friction of a head with three or more drives. However, these non-powered idler wheels tend to plug with debris and cease to rotate, thereby contributing to greater friction and loss of performance.
It is also known in the prior art that heads using multiple tree feeding drives can be configured to provide anti-slip against the tree as it is feeding through the head by using a flow-divider principle. For example, International Patent Application publication no. WO 99/41972 (Pinomaeki) shows how each of two main driving wheels having their respective hydraulic motors connected in series to corresponding hydraulic motors mechanically connected to each other and to a third driving wheel can limit slip of the main drive wheels. However, using this design adds cost and complexity to the drive system without improving the feed force or feed speed over simple two wheel drive systems. This design simply forces all three wheels to drive the tree at the same relative speed, thereby eliminating slip unless all three wheels slip. If all three wheels slip, there is no appreciable improvement.
Also in the prior art, it is known that harvesting certain tree species, for example eucalyptus, requires the bark to be removed from the stem almost immediately after felling. If the bark is allowed to remain on the stem for any length of time, it becomes very difficult and expensive to remove. Most tree harvesting heads used in these applications are equipped with special feeding wheels to help strip the bark from the stem. Typically it requires a minimum of three and up to seven or more full length tree-feeding passes to remove the bark. The more passes required, the more inefficient the operation and the more damage is done to the tree fibres.